Vitamin D in Winter: Benefits, Deficiency, and How to Get Enough

• 25 September 2026, by SANUSq Research team
• 16 min reading time

From October to March, the sun over most of Europe sits too low for your skin to make much vitamin D — which is why winter is when a shortfall quietly takes hold. This guide sets out why the vitamin D winter happens, how to check it for yourself, what vitamin D actually does, the facts that hold up and the myths that don't, and how to top up sensibly with vitamin D3 and K2.

Written by SANUSq Research Team | Medically reviewed by Dr Ajay Kumar, MD
Published: 25 September 2026 | Updated: 25 September 2026

Why winter is the danger zone

Most of our vitamin D isn't eaten — it's made in the skin when UVB sunlight reaches it. But UVB only reaches the ground in useful amounts when the sun is high in the sky. From roughly October to March, at our latitudes, the sun sits too low for its UVB rays to make meaningful vitamin D at all — so however much time you spend outdoors on a crisp, bright winter's day, your skin simply can't produce it. The body draws on its summer stores, but for many people those run down over the colder months, and blood levels typically bottom out in late winter and early spring. That's why the time to think about vitamin D is the start of the season, not the end of it.

How far north is "northern"? Further south than most people assume. Marseille sits at 43°N, Lyon and Geneva at 46°N, Paris at 49°N and Brussels at 51°N — all well inside the zone where the vitamin D winter begins around October. The further north you live, the longer it lasts.

The shadow rule

There's a simple way to see the vitamin D winter for yourself. Step outside on a sunny day and look at your shadow: if your shadow is longer than you are tall, the sun is too low for your skin to make much vitamin D. It's a rule of thumb rather than a lab measurement, but it tracks the sun's angle well. By late October, even at midday, your shadow in Paris is almost twice your height. And sunshine through a window doesn't count — glass blocks UVB.

What vitamin D actually does — and why it matters

Vitamin D behaves more like a hormone than a typical vitamin, and receptors for it sit on cells throughout the body. Its best-established job — the foundation of vitamin D bone health — is managing calcium: vitamin D contributes to the normal absorption and use of calcium and phosphorus, to normal blood calcium levels, and to the maintenance of normal bones and teeth. It also contributes to normal muscle function and to the normal function of the immune system. That breadth is where its importance genuinely lies — but it's also why vitamin D gets over-claimed, so it's worth separating the solid from the speculative.

Why deficiency is so common — and who is most at risk

Winter is the biggest factor, but modern life adds to it: days spent indoors, covering up, and sunscreen in summer (sensible for skin protection) all reduce how much vitamin D the skin makes. The result is one of the most common nutritional shortfalls there is. It isn't spread at random, though. People most likely to run low include:

• Older adults, whose skin makes vitamin D less efficiently.
• People with darker skin, because melanin filters UVB.
• Anyone who rarely goes outside or covers up for cultural, work or health reasons.
• People carrying extra weight, as vitamin D is held in fat tissue.
• Pregnant and breastfeeding women, whose needs are higher.

If two of these apply to you and it's February, the chance of a low level is high.

Vitamin D deficiency signs: what to look for

Deficiency is often quiet, and the signs people notice are non-specific — which is exactly why it's so easily missed. Common clues include persistent tiredness and low energy, frequent colds, bone or muscle aches, and low mood over the winter months. Two more are easy to overlook: gradual hair thinning and slow wound healing. Because these overlap with so many other things, they're a prompt to ask for a simple blood test (25-hydroxyvitamin D) rather than a diagnosis in themselves — testing is the only way to know your level for sure.

Vitamin D and immunity

The immune connection is one of the better-studied areas, and it's where honest nuance matters most.

The practical message: correcting a shortfall matters; piling more on top of a healthy level doesn't. We look at this in detail in vitamin D and respiratory infections.

Fatigue, winter and vitamin D

It's a good reason to keep an eye on your status through the darker months — with one condition kept firmly in mind: the participants were deficient to start with. Vitamin D is not a general energy supplement for people whose levels are already fine.

Vitamin D, the brain and the heart

Here the evidence takes the same shape twice: interesting links in observational studies that shrink when tested directly. Low vitamin D is associated with a higher risk of cognitive decline, and one large observational study found that older adults taking vitamin D were less likely to develop dementia (Ghahremani et al. (2023) — but observation can't prove cause, and a randomised trial found no clear effect. On the heart, population data links low vitamin D with cardiovascular risk, yet large trials in mostly-replete people found little or no benefit.

The partner nutrients: vitamin D3, K2 and magnesium

When the sun stops making vitamin D for you, what you take matters more — and three details make the difference.

The form: D3. Vitamin D3 (cholecalciferol) is the same form your skin makes in sunlight, and it raises blood levels more effectively than the plant form, vitamin D2.

The partner: K2. Vitamin D helps bring calcium in; to build that calcium into bone, your bone-forming cells make a protein called osteocalcin, which only becomes active once vitamin K has switched it on. That's why vitamin K also contributes to the maintenance of normal bones — and why, in winter, when you rely on a supplement for vitamin D, it makes sense to give it its partner. We cover this pairing in depth in our guide to vitamin K2 and D3.

The helper: magnesium. The enzymes that convert vitamin D into its active form depend on magnesium, so low magnesium can leave vitamin D stored but under-used.

Liposomal vitamin D: what the form changes

Vitamin D is fat-soluble, so its absorption normally depends on some fat in the meal and on good digestion. Liposomal forms wrap the vitamin in tiny lipid spheres, with the aim of making absorption less dependent on what you've eaten and how well you digest it. The honest nuance: direct comparisons between forms are still limited. For someone who digests well and takes vitamin D with a meal containing fat, a standard oil-based form works perfectly well — liposomal is a convenience and consistency choice, not a magic upgrade.

How to get enough this winter — sensibly

A practical approach beats both neglect and mega-dosing:

• Sunlight, in moderation, in the warmer months: short, regular exposure helps build stores, without burning.
• Food: oily fish (salmon, mackerel, sardines, herring), egg yolks, liver and fortified foods all contribute. Two portions of oily fish a week is the most useful habit — though diet alone rarely covers a winter shortfall.
• A supplement from October to March, especially if you're in one of the higher-risk groups: vitamin D3, ideally with K2, taken daily or weekly rather than in large occasional doses. Start in early autumn rather than waiting until February, when levels are already at their lowest.
• Don't overdo it: vitamin D is stored in the body, and very high doses over long periods can raise blood calcium. The European Food Safety Authority sets a tolerable upper intake for adults of 100 µg (4,000 IU) a day from all sources. If in doubt, a blood test lets you adjust rather than guess.

Our Liposomal Vitamin D3 + K2 provides 1,000 IU (25 µg) of vitamin D3 and 45 µg of vitamin K2 as MK-7 in each 1 ml daily serving, taken as drops. One 60 ml bottle lasts two months, so three bottles cover the vitamin D winter from October to March — which is exactly what our three-bottle bundle is for.

Common vitamin D myths — and the facts that correct them

• "A sunny winter's day tops me up." Not at our latitudes between October and March — the sun is too low, however bright it looks. Check your shadow.
• "Vitamin D prevents colds." The benefit is modest, uncertain, and concentrated in people who were deficient to begin with — correcting a shortfall, not a shield.
• "More is better." Vitamin D accumulates, so the aim is a healthy level, not the highest number you can reach.
• "Vitamin D replaces the sun." Daylight also acts on your body clock and mood in ways a supplement doesn't — winter walks still matter, even when your skin isn't making vitamin D.

Frequently asked questions

Why is vitamin D deficiency so common in winter?

Because from roughly October to March, at our latitudes, the sun is too low for enough UVB to reach the ground, so the skin makes little or no vitamin D. Summer stores run down through the season, and levels typically reach their lowest point in late winter.

How can I tell if the sun is still enough?

Use the shadow rule: if your shadow is longer than you are tall, the sun is too low for your skin to make much vitamin D. At our latitudes that's true for most of the day from October to March, even in bright sunshine.

Should I take vitamin D in winter?

For most people at our latitudes it's a reasonable choice, and clearly worthwhile if you're in a higher-risk group — older age, darker skin, little time outdoors, or carrying extra weight. If you're unsure, a blood test lets you adjust rather than guess.

How do I know if I'm deficient?

The signs — tiredness, frequent colds, aches, low mood, hair thinning — are non-specific, so the only reliable way is a blood test for 25-hydroxyvitamin D. Deficiency is most likely in late winter and in the higher-risk groups above.

Should I take vitamin D3 with K2 and magnesium?

It's a sensible combination. Magnesium is needed to activate vitamin D, and vitamin K activates osteocalcin, the protein that builds calcium into bone. Make sure your diet provides enough magnesium, and choose a vitamin D3 supplement paired with K2.

Do I need a high-dose vitamin D3 K2, such as 5,000 IU?

More is not automatically better. EFSA's tolerable upper intake for adults is 100 µg (4,000 IU) a day from all sources combined, so a 5,000 IU daily product goes above that and should only be used on professional advice, ideally after a blood test. A moderate daily dose, taken consistently through the winter, is the sensible default.

How do I take vitamin D3 K2 drops?

Follow the dose on the label. For our Liposomal Vitamin D3 + K2 that's 1 ml a day (1,000 IU of D3 and 45 µg of K2 as MK-7), measured with the dropper, at any time of day — many people take it with breakfast so it becomes a habit. If you take a blood-thinning medication such as warfarin, speak to your doctor before starting any vitamin K supplement.

Can vitamin D prevent colds, dementia or heart disease?

Mainly by correcting a deficiency, and only modestly. It slightly lowers respiratory-infection risk (most in deficient people) and is linked observationally with lower dementia and heart risk, but large trials have generally not shown it protects people who already have enough. Fixing a genuine shortfall is worthwhile; expecting it to work like a drug is not.

References

1. Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Journal of Clinical Endocrinology and Metabolism. 1988;67(2):373–378. PMID 2839537
2. O'Neill CM, Kazantzidis A, Ryan MJ, et al. Seasonal changes in vitamin D-effective UVB availability in Europe and associations with population serum 25-hydroxyvitamin D. Nutrients. 2016;8(9):533. PMID 27589793
3. Cashman KD, Dowling KG, Škrabáková Z, et al. Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition. 2016;103(4):1033–1044. PMID 26864360
4. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. PMID 28202713
5. Nowak A, Boesch L, Andres E, et al. Effect of vitamin D3 on self-perceived fatigue: a double-blind randomized placebo-controlled trial. Medicine (Baltimore). 2016;95(52):e5353. PMID 28033244
6. Ghahremani M, Smith EE, Chen HY, et al. Vitamin D supplementation and incident dementia: effects of sex, APOE, and baseline cognitive status. Alzheimer's & Dementia (Amst). 2023;15(1):e12404. PMID 36874594
7. Thompson B, Waterhouse M, English DR, et al. Vitamin D supplementation and major cardiovascular events: D-Health randomised controlled trial. BMJ. 2023;381:e075230. PMID 37380191
8. Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition. 2012;95(6):1357–1364. PMID 22552031
9. Uwitonze AM, Razzaque MS. Role of magnesium in vitamin D activation and function. Journal of the American Osteopathic Association. 2018;118(3):181–189. PMID 29480918

Read more on the blog

• Vitamin K2 (and D3): the key to where your calcium goes →
• Vitamin D and respiratory infections: what the evidence shows →
• Why you should take magnesium with vitamin D →

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We wish you good health on your terms.
The team at SANUSq.

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